This invention relates generally to permanent magnet motors and generators and, more particularly, to a highly efficient sealed AC radial flux motor/generator having a ring of permanent magnets mounted on the cylindrical surface of an inner rotor and having a ring of electromagnets mounted on an outer stator such that outwardly facing surfaces of the permanent magnets rotate in close proximity to the inwardly facing surfaces of the ring of electromagnets.
Variable speed electric motors of different types have been employed in a variety of applications over the years, most recently in electric vehicles. One such motor is the series DC motor, which employs brushes and wound coils. These motors suffer from low peak power densities. Thus, they are heavy and unresponsive; they are not sealed since air flow through the motor is required to cool the rotor; and they cannot run in reverse. Furthermore, their speed is difficult to control under varying load conditions, and they have no power regeneration capability.
Three-phase asynchronous AC induction motors are typically powered by a DC battery pack coupled to a DC/AC inverter and associated pulse width modulation circuitry to achieve variable speed control. These motors are characterized by heavy weight, low torque, and power factor losses. Power electronics are employed to receive a DC voltage supplied by a battery pack and produce an AC voltage whose waveform frequency is modulated so that the motor's speed may be varied. These motors offer good speed control, are brushless, can be sealed, can run in reverse, and provide power regeneration. However, AC induction motors exhibit low starting torque, which means they usually require a gear box, have power factor losses, are typically more expensive than series DC motors, and the controller required to vary the motor's speed is large, complex and expensive as it must handle the slippage between the input current and the rotational speed necessary for such an asynchronous motor to operate.
Brushless DC motors, a more recent development, are lighter, more powerful, produce more torque, and are typically more efficient than comparably sized AC induction motors. These motors use permanent magnets mounted on the rotor, obviating the need for brushes as no power need be supplied to the rotor. Power to the electromagnets mounted on the stator is typically controlled by power electronics that sense the position of the rotor permanent magnets with Hall sensors and use six-step commutation and pulse width modulation to control the electromagnets. These motors offer good speed control, power regeneration, are brushless, are sealed to protect the internal components from dirt and foreign particles, are reversible, are lighter in weight, are more powerful, are more efficient than series DC motors, and exhibit higher torque than AC induction motors.
The most recent development in motor art is the permanent magnet AC motor which is virtually identical to the brushless DC motor except that it uses more precise sinusoidal control than six-step commutation, with the result that the motor delivers more torque, especially at the higher speeds at which it is capable of operating. Sinusoidal controllers typically require encoders or resolvers, rather than Hall sensors, to more accurately signal the position of the rotor electromagnets so that power can be applied to them as a smooth sinusoidal waveform rather than as a stepped waveform. Thus, the essential difference between a brushless DC motor and a permanent magnet AC motor lies not in the motor itself but in how it is driven, either sinusoidally or by step commutation, and in the required sensing. As a broad class, brushless DC (BLDC) motors and permanent magnet AC (ACPM) motors are appropriately referred to simply as permanent magnet motors. Unlike asynchronous AC induction motors, there is no slippage between the input current and the motor speed and so an ACPM motor controller is lighter, less complex, and less expensive than the controllers used for AC induction motors.
A permanent magnet motor, whether a BLDC or an ACPM motor, is usually one of two types. The first type is a radial flux motor in which the magnetic flux field in the air gap between the rotating permanent magnets and the stationary electromagnets is produced in a radial direction from the center of the motor to the outside thereof, and vice versa. The second type is an axial flux motor in which the magnetic flux field in the air gap between the rotating permanent magnets and the stationary electromagnets is produced in an axial direction parallel to the axis of rotation of the rotor. Both types of motors offer good magnetic structures, but radial flux motors have a mechanical advantage in that the attractive and repulsive forces between the permanent magnets and electromagnets are carried by the rotor and stator flux rings in tension or compression, while those forces are carried in axial flux motors by the rotor and stator plates in bending, thus requiring a heavier structure and heavier bearings.
Permanent magnet AC and DC motors are known in the prior art. U.S. Pat. No. 5,689,147 to Kaneda et al. is directed to a brushless motor having a toothed stator core. U.S. Pat. No. 5,903,082 to Caamano is directed to an axial flux motor/generator having a laminated amorphous metal core. U.S. Pat. No. 5,918,360 to Forbes et al. is directed to a radial flux motor having individually-wound laminated electromagnetic cores. U.S. Pat. No. 6,700,279 to Uchiyama et al. is directed to a brushless motor in which the electromagnet cores are teeth on the stator flux ring, U.S. Pat. No. 6,384,496 to Pyntikov et al. is directed to a multiple-magnetic-path motor in which electromagnets are arranged in pairs rather than individually. U.S. Pat. No. 6,617,746 to Maslov et al. is directed to a radial flux motor in which electromagnets are arranged in pairs rather than individually. U.S. Pat. No. 6,717,323 to Soghomonian et al. is directed to a radial flux motor in which electromagnets are arranged in pairs rather than individually. U.S. Pat. No. 6,727,629 to Soghomonian et al. is directed to a radial flux motor in which electromagnets are arranged in pairs rather than individually. U.S. Pat. No. 7,541,711 to Adaniya et al. is directed to a radial flux motor in which the electromagnet coils are wound on sharp-cornered stator teeth. U.S. Pat. No. 7,898,134 to Shaw is directed is directed to a radial flux motor that utilizes a double rotor rather than a stator flux ring.
Representative of the prior art cited above is the motor/generator 1000 illustrated in FIG. 1 of the present drawings that includes a stator flux ring 1100, a plurality of teeth 1200 formed to protrude inwardly from stator flux ring 1100 serving as the cores of stator electromagnets like electromagnet 1401 of FIG. 2, and a like plurality of shoes 1300 positioned at the inward ends of the teeth 1200 for the purpose of optimally diffusing the magnetic flux field produced by the stator electromagnets 1401. Stator flux ring 1100, teeth 1200, and shoes 1300 are typically formed as a single laminated component using steel plates stamped from a stack of steel plates. The use of laminated steel plates minimizes eddy currents and consequent iron losses in the finished component and also provides the mechanical strength required to resist the rotational forces applied at the shoes 1300. Stator flux ring 1100 is conventionally mounted to a stator plate or directly to the motor case so that the shoes 1300, electromagnets 1401, and stator flux ring 1100 are able to transfer those rotational forces to the external body to which the motor is mounted.
Each of the teeth 1200 is typically wound with insulated wire 1400 to form the electromagnet 1401 shown in cross-section in FIG. 2. As illustrated, the windings 1400 contact the square corners 1202 of the teeth 1200, thus leaving a considerable air gap 1450 between the windings 1400 and the laminated core material that forms each of the teeth 1200. Electromagnets 1401 can be formed with an air core only, but in that case would produce a much lower magnetic force. Ferrous cores serve to greatly increase the magnetic force produced by any electromagnet. Thus, the magnetic force produced by prior art electromagnet 1401 is limited due to the presence of the air gap 1450. Because the physical size of electromagnets 1401 is limited in any given motor design, the size of copper wire 1400 is also limited, thus increasing the resistive losses in the windings. In addition, it is difficult to mechanize the winding of teeth 1200 in the presence of shoes 1300, so winding is typically accomplished manually, at much higher cost.
The theory of operation of the prior art motor/generator 1000 of FIG. 1 is well understood by those skilled in the art. The magnetic flux field produced by each of the electromagnets 1401 is selectively switched on and off by an electronic controller, based on the relative position of the rotor with respect to the stator. This magnetic flux field extends through the shoes 1300 and the air gap 1800 and interacts with the magnet flux field produced by the permanent magnets 1600, which are either mechanically or adhesively mounted on the rotor flux ring 1700, thereby producing rotational motion of the rotor body 1750 and of a drive shaft to which it may be connected. The space 1850 between the finish cover 1990 and the stator 1100 can be filled with a heat conducting epoxy or left empty.
The present application is directed to a radial flux permanent magnet AC motor/generator having a magnetic and mechanical structure that is unlike any of the representative prior art motors described above.
In accordance with the illustrated preferred embodiment of the present invention, a radial flux permanent magnet AC motor/generator employs a flat circular stator plate having a plurality of electromagnets mounted in a ring pattern on an inner face thereof. A circular flux ring fabricated of powder metal is permanently mounted to the stator plate outside the ring of electromagnets. Permanent magnets are mounted in a ring pattern on the outer cylindrical surface of a steel rotor. The stator plate and the rotor are axially and diametrically aligned such that the ring of permanent magnets rotates in close proximity to and inside the ring of electromagnets and such that the magnetic flux field across the air gap between the permanent magnets and electromagnets is in the radial direction. The electromagnets utilize powder metal cores which minimizes eddy currents and resultant heat losses, to permit the use of heavier gauge copper windings to minimize resistive power losses and attendant heat, and to allow the use of higher frequency drivers to further increase the magnetic force produced by the electromagnets.
The radial flux permanent magnet AC motor of the present invention utilizes electromagnets whose cores are not simply tooth extensions formed along the inner edge of a laminated flux ring, in accordance with the prior art. Instead, the electromagnets of the present invention are separately formed units that can be more easily wound, using automatic machines, and that are made of powder metal that can be shaped to have rounded corners and flat sides that permit the use of heavier gauge windings and facilitate heat removal. Further, by so shaping the electromagnet cores, the usual air gaps between a core and the windings thereon are eliminated, resulting in the production of a stronger magnetic field.
The electromagnets are anchored to resist the rotational forces produced by the motor through the use of a structural epoxy material filling the space between each of the electromagnets and the stator surface on which the electromagnets are positioned. The epoxy material in turn transfers the rotational forces to non-metallic anchor plates that are mounted to the stator midway between each of the electromagnets. This is unlike prior art motors in which the electromagnet cores are tooth extensions of the flux ring and rotational force is transferred through the core to the flux ring.
The precise location of the rotor is monitored through the use of a conventional encoder or resolver, and conventional sinusoidal control circuitry is employed to direct operating power and to control the speed of the motor.
The present motor exhibits higher power density and lighter weight than prior art permanent magnet AC motors or brushless DC motors. It is also more efficient, resulting in low heat generation, the small amount of which may be quickly conducted away by means of a liquid coolant circulated through tubes imbedded in the stator and positioned under the electromagnets. Alternatively, heat may be conducted to an outer surface of the motor that is air-cooled or by connecting the motor to a large heat sink such as the body of a vehicle using metallic heat conducting fasteners or a heat strap.
The present motor may be driven to 200% of its sustainable continuous current to thereby double the output horsepower for short periods of time. It is reversible, is capable of power regeneration, and offers good speed control. By employing a large diameter rotor and many electromagnet pole pieces, and by increasing the power of the electromagnets through the use of heavier gauge windings and more flux magnifying material in the cores, the present motor exhibits up to 200% more starting torque than prior art brushless DC and permanent magnet AC motors, thus eliminating the need for a gear box or clutch in electric vehicle applications. Since the rotor components are individually assembled and the electromagnet coils are machine wound, the present motor can be manufactured at a cost saving over prior art motors.
In summary, the AC motor/generator of the present invention 1) utilizes electromagnet cores fabricated of a high-performance powder metal material to increase their performance over prior art laminated steel cores; 2) utilizes electromagnets wound with heavy gauge copper wire to minimize resistive losses; 3) eliminates any air core from the electromagnets thereof to thereby increase the magnetic force produced by the electromagnets; 4) utilizes electromagnets having a flat surface that is interfaced to the stator surface to provide improved heat conduction from the electromagnets to the stator; 5) utilizes separately formed cores and shoes in order to facilitate machine winding of the electromagnets; 6) utilizes electromagnetic cores that are separate from the flux ring and shoes to further enable machine winding of the electromagnets; 7) utilizes a structural epoxy material in combination with non-metallic anchor plates to anchor the electromagnets to the stator; and 8) utilizes wedges to hold the permanent magnets to the rotor surface to provide resistance to rotational forces, thereby eliminating the prior art use of screws that penetrate the permanent magnets, displacing some of the magnetic material thereof and reducing their magnetic field strength.